The demand for making apparatuses for measurements of linear dimensions featuring wide measurement ranges and high repeatability and accuracy is increasingly felt, also in view of the development of flexible manufacturing systems.
In particular, this demand arises for checking parts during grinding machining. In this case, in addition to the already mentioned requirements, there are those of small overall dimensions, high sturdiness, working reliability in harsh environment, quick response.
The requirement of having small overall dimensions is important for permitting the use of the apparatus without any need of performing heavy fitting in the grinding machine and for permitting easier part loading and unloading operations. During these operations the apparatus is generally moved away from the operating position by means of a slide or a rotating arm, but it is evident that small overall dimensions of the apparatus are of advantage on this regard, too.
At the same time, in order to obtain wide measuring ranges and high accuracies, it is necessary to use transducers and mechanical structures involving considerable minimum overall dimensions.
Patent application GB-A-2057688 discloses an apparatus that does not appear to be fit for providing a quite satisfactory compromise among the above mentioned contrasting requirements. This known apparatus comprises two vertical slides, arranged side by side, whose position is detected by corresponding wide-range digital measuring units. Each slide supports a rotating arm having a feeler adapted to contact the part whose diameter must be measured. The position of the rotating arm is detected by a small-range transducer, the signal of which controls, through a feedback loop, a motor actuating the slide by means of a threaded stem coaxial with the same slide and the motor. This control system tends to nullify the values of the signals of the two small-range transducers, so that the value of the part diameter is obtained by combining the signals of the two wide-range digital measuring units.
The known apparatus does not appear capable of guaranteeing very high repeatabilities and accuracies. Apart from the errors due to the obtainment of the part measurement only depending from the signals of the digital measuring units, the arrangement of the slides, of the digital measuring units and of the part is such as to necessarily involve not small errors. For example, the unavoidable errors due to the non-straightness and to the plays in the guides heavily affect the digital measuring units, that are arranged at a considerable distance from the vertical plane passing through the part axis, while this distance--in view of the well-known Abbe's principle --in the optimum conditions should be null.
Moreover, the known apparatus has a structure very open i.e. poorly protected, and that however is difficult to protect by envelopes, sealing devices, etc.
The size in the vertical direction is considerable, too, notwithstanding the use of slides and motors arranged side by side and of distinct digital measuring units.
U.S. Pat. No. 3,400,573 shows an apparatus that has many features--in particular the control system--similar to those of the apparatus previously described, but that uses a single wide-range transducer, and pneumatic sensors as measuring heads. Among the limits of this apparatus there is, in particular, a considerable size along the vertical direction.
U.S. Pat. No. 3,996,669 discloses a measuring apparatus comprising two vertical slides arranged one over the other, each of which is associated with a linear position transducer and carries a measuring head. The slides are actuated by relevant motors through threaded stems and cooperate with guide rods stationary with respect to a frame. This apparatus features a considerable size along the vertical direction and the arrangement of the linear position transducers and of the measuring heads, that are situated at opposite sides with respect to the driving stems and the rods for guiding the slides, may bring about considerable measurement errors. This apparatus, too, has a very open structure, that is not suitable for the provision of reliable protection and sealing devices.
The U.S. Pat. No. 3,996,669 patent also discloses a measuring apparatus having a single slide carrying a measuring head, and with the same arrangement of the linear position transducer and of the head.